Carb Heat

I am in process of installing a engine in a previously flying RV3 purchased
less engine. My airbox is a very small fiberglass with a round auto airfilter
that when installed on carb will be in front and slightly above the carbs
bottom. There is a lever and valve but no clear way that I can see to obtain
carb heat. Do RV3's use the small tube that clamps to the exhaust crossover? It looks like that when the cable for carb heat is pulled it takes
air thru the filter and not the ram air from the front opening of the airbox.
I am totally lost here any advice or photo's appreciated.

Thanks
Jim Miller

Earlier RVs did not use a tube to direct exhaust pipe heat to the carb intake. They simply blocked off the ram air and used plenum air. This air is warmer than the ram air intake and generally works fine for Lycomings. There are even a few Standard Certificated aircraft that used this method.

I have a carb ice detector in my Cessna 180. It really works well. It'll detect ice before there's any outward evidence of it and tell me when it's cleared.

http://www.aircraftspruce.com/catalog/inpages/icedetect2.php

It has a probe that fits in the carb itself.

Dave

Mel, Dave
Thanks for the replies. That clears up a major question. Did the early RV-3's
have any problems with this carb heat system? I have flown my 0235 Colt
since 93 or 94 and never had occasion to use carb heat.

With my airbox the only time the air is filtered is when carb heat is on
which is almost never. Isn't that hard on the engine? Does the newer
airboxes provide filtered air all the time or at least when carb heat is OFF.

jim miller said:

Mel, Dave
Thanks for the replies. That clears up a major question. Did the early RV-3's
have any problems with this carb heat system? I have flown my 0235 Colt
since 93 or 94 and never had occasion to use carb heat.

Click to expand...

Not that I'm aware of. I flew my -6 for about 4 years before adding the additional "plumbing" and never had a problem.
Actually the early -3s and -4s had no filter at all. The original cowlings were very tight.

RV3 carb heat

I flew my RV3 for 12 yrs without a carb heat-muff. Normal ram-air goes through a Brackett filter, but the alternate-air intake came from the 'warmer' air in the plenum around the carburator. Seemed to work fine - I only got carb-ice a couple times when running up before 1st takeoff of the day, and only when it was wet and cold (Pacific NW style). Well, then I found out that carb-ice can be undetected until ~80% of the throat is blocked. So I bit the bullet, made room in the tight cowl, and put on a heat muff. While I was at it, I put on a carb-throat temperature sensor. When OAT is ~40deg, I often see carb throat temp in the teens (deg-F). Ok, so Lycomings are unusually resistant to carb-ice, as the carb is mounted on the hot oil sump. Good. But when the Stuff Hits The Fan, and the Fan Stops, I want a heck of a lot of hot air hitting my carb throat. Moral to Story: put on a muff and use carb heat at low RPM's, just like we were taught before they cutoff our shirttails!

- Steven
700+ RV3 hours
(two more last Monday!)

carb heat

Lycoming operators handbook DOES NOT recommend routine use of carb heat. Lycoming vs Continental, two VERY different scenarios.

jrs14855 said:

Lycoming operators handbook DOES NOT recommend routine use of carb heat. Lycoming vs Continental, two VERY different scenarios.

Click to expand...

If it doesn't say it will hurt anything, i'd rather use it. I've seen carb ice in the air in my -6, flying here in the Pacific Northwet. If you're in a warmer and dryer climate, I can see an argument for not installing carb heat, but in colder/wetter regions you can, and likely will, get carb ice from time to time.

brackett air filter

ok, here's a pic of my airplane (first time I've tried up put a photo on here - hope it comes out). In case the photo doesn't take, it's an externally mounted square Brackett filter. I built the airplane in the '80s when everyone was scooping unfiltered air ... which I thought was crazy, so I came up with this. Fairly innovative in it's day ... pretty clunky now! Works great, though (probably lose ~5kts on the top end!)

Steven
Thanks for posting photo. My Cowling and plane is from 1989 and has the
small round hole for ram air. I haven't hung the engine yet and have no
feel for how much space inside I will have. Would one of van's current
airboxes with the round filter work on the RV3? What is the current RV3B's
being built use for airbox/filter and could they be modified to fit inside a
cowling of 1989 vintage/

Thanks
Jim Miller

RV3 intake air box

Jim - Here's a shot of my carb intake/alt-air box, and of the lower cowl. I bought the finishing kit ~1987, so you may have the same. The air-box fits very snug along the center bottom of the cowl - I doubt if there is room for anything else in there without significant modifications. That's why I mounted the filter externally. If you have fuel injection, I can't comment, as I only have experience w/carbs. Note the four-stack Vetterman exhaust: his work is gorgeous and fits perfectly without any mods to the cowl (the red hi-temp silicon merely holds the heat-reflector pad a little more securely to the cowl - the silver sheet tends to peel off if the edges aren't secured).

Steven
That's exactly the airbox I have except the builder put a round auto air
filter in the area here you have the carb heat input duct filtering only
plenum air with no exhaust pipe/heat muff. I would like to try and remove
the airfilter and restore the top part like your photo shows and if possible
build a thin metal box to hold a brackett filter element but remain inside
cowling which is probably not possible. I may have a little extra room with
the 0235 versus 0320's. I could cut back on the forward fiberglass of
the airbox about one inch and make a round box if that would help. The brackett filter sponge is only about 3/4" thick. Do you think something like this is possible?

carb heat

Jim - If you have a crossover exhaust, then what you're trying to do would be very difficult. There's just no room up there for a filter box with an in/out SCAT tube, and still stay far enough away from those hot pipes. The pipes are too close together to mount the carb heat muff on the crossover (I know, I tried.)

What I ended up doing (when I had crossover pipes) was simply use a short stove-pipe SCAT tube, straight up from the alt-air intake to within 2" or so of the crossover. When alt-air was selected, it would draw even warmer plenum air from around those crossover pipes. Secure a little gascolator-type mesh screen on the alt-air intake under the SCAT tube to keep junk from falling in.

(IMHO: Filtering of alt-air is a good idea, but is it necessary? Hot alt-air is useful for low RPM operation only when OAT is, say, less than ~50degF or so, to deter carb-ice. I get a 100RPM drop now that I'm using that itty-bitty Vetterman heat muff! More soot in the pipes too. I have not looked at my plugs yet since putting the muff on - they have been perfect for the 700+ hours I've run the engine, but if they are soot-covered, I will probably take the muff off and go back to what I had before: plenum air taken in near the pipes.)

However, if you switch to a 4-pipe Vetterman system, you've got all the room in the world above the carb alt-air intake. You could fabricate a box for the filter, mount it on the oil-pan bolts or the engine case bolts, or both.

- Steven
700+ RV3 hours
(and counting)

JIm,
I have the banjo box from vans attached to my 0-290 pwrd -3.. Will post a few pics of the lower cowling, bracket and filter assy.. should be home later tomorrow..
Jeff

Jeff
Thanks. That will be a big help. One thing I have learned from Steven's photo is the carb on the 0320 is a lot further forward that on my 0235. This will give me more room to work with
as I come forward with a box. It will be interesting to see your photo's and how they compare with Steven's 0320 and my 0235.

Thanks
Jim Miller